Coloured dark blue (officially "Corporate Blue", Pantone 072) on the Tube map, it is the fourth-busiest line on the Underground network with over 210 million passenger journeys in 2011/12. It is partly a deep-level line, with a number of surface sections, mostly in its westernmost parts. It is named after Piccadilly, the street under which it runs between Hyde Park Corner and Piccadilly Circus. Some of its stations are shared with the District line (between South Kensington and Ealing Common) and some are shared with the Metropolitan line (from Rayners Lane to Uxbridge). It is the second-longest line on the system (after the Central line) and runs to the system's second-largest number of stations (after the District line).

For detailed histories of the Great Northern, Piccadilly & Brompton Railway (GNP&BR), the Great Northern & Strand Railway (GN&SR), and the Brompton & Piccadilly Circus Railway (B&PCR), see Great Northern, Piccadilly and Brompton Railway.

The GNP&BR was formed from the merger of two earlier, but unbuilt, tube-railway companies taken over in 1901 by Yerkes' consortium: the Great Northern & Strand Railway (GN&SR) and the Brompton & Piccadilly Circus Railway (B&PCR). The GN&SR's and B&PCR's separate routes were linked with an additional section between Piccadilly Circus and Holborn. A section of the District Railway's scheme for a deep-level tube line between South Kensington and Earl's Court was also added in order to complete the route.

On 30 November 1907, the short branch from Holborn to the Strand (later renamed Aldwych) opened; it had been planned as the last section of the GN&SR before the amalgamation with the B&PCR. In 1905 (and again in 1965), plans were made to extend it the short distance south under the River Thames to Waterloo, but this never happened. Although built with twin tunnels, single-track shuttle operation became the norm on the branch from 1918 on, with the eastern tunnel closed to traffic.

On 10 December 1928, a rebuilt Piccadilly Circus station was opened. This included a sub-surface booking hall and eleven escalators, replacing the original lifts, and was the start of a renovation of the whole railway, including a comprehensive programme of station enlargement.

From the 1920s onwards there had been severe congestion at the line's northern terminus, Finsbury Park, where travellers had to change on to trams, buses, or London and North Eastern Railway (LNER) main line trains for destinations in north and northeast London. There had been deputations made to Parliament asking for an early extension of the line either toward Tottenham and Edmonton, or toward Wood Green and Palmers Green.

The early 1930s was a time of severe recession, and government capital was made available in order to relieve unemployment. The chief features of the scheme were an extension northwards from Finsbury Park to Cockfosters. It was also planned to build a station between Manor House and Turnpike Lane at the junction of Green Lanes and St Ann's Road in Harringay, but this was stopped by Frank Pick, who felt that the bus and tram service at this point was adequate. However, a 'Ventilation station', in similar architectural style to tube stations of the time was provided at the site, and is visible today. There was also some opposition from the LNER to the line. The extension began from Finsbury Park to a point a little south of Arnos Grove. The total length of the extension is 12 km (7.5 mi): it cost £4 million to build and was opened in sections as follows:

Powers to link with existing tracks west of Hammersmith were obtained in 1913. A Parliamentary report of 1919 recommended through running to Richmond and Ealing. By the end of the 1920s, the priority had shifted to serving the areas around Hounslow and north and west of Ealing. The outcome involved taking over the inner pair of tracks between Hammersmith and Acton Town as a non-stop service, while the Metropolitan District Railway would continue to provide the stopping service on the outer pair of tracks.[4] Construction of the linking sections started in 1930, and the services opened as follows.

to Uxbridge: the District Railway had operated services to Uxbridge since 1910. The District services were taken over by the Piccadilly line:

to Hounslow: the line from Acton Town was quadrupled to Northfields on 18 December 1932 and the Piccadilly line was extended:

9 January 1933: to Northfields

13 March 1933: to Hounslow West, in conjunction with the eastern extension to Enfield West.

These eastward and westward extensions feature Modernist architecture at their stations, many of them designed by Charles Holden, who was inspired by examples of Modernist architecture in mainland Europe. This influence can be seen in the bold vertical and horizontal forms, which were combined with the use of traditional materials like brick.[5] Many of these Holden-designed station are listed buildings.

During the planning stages of the Victoria line, a proposal was put forward to transfer Manor House station to the new line, and also to build new "direct" tunnels from Finsbury Park to Turnpike Lane station, thereby cutting the journey time in and out of central London. This idea was eventually rejected due to the inconvenience to passengers that would have been caused during rebuilding, as well as the costs of the new tunnels. Even so, the Piccadilly line was affected at Finsbury Park by the construction of the Victoria line. The westbound service was redirected through new tunnels, to give cross-platform interchange with the Victoria line on the platforms previously used by the Northern City Line. This work was completed in 1965, and the diversion came into use on 3 October 1965, three years before the opening of the first stage of the Victoria line.

In 1975, a new tunnel section was opened to Hatton Cross from Hounslow West. Hounslow West became a tunnel section station. In 1977, the branch was extended to Heathrow Central. This station was renamed Heathrow Terminals 1, 2, 3 in 1984, with the opening of a one-way loop serving Heathrow Terminal 4, south of the central terminal area. To reflect the demolition of Terminal 1 at the end of June 2015, it was renamed again as Heathrow Terminals 2 & 3, although the station signage retains reference to the demolished terminal.

From 7 January 2005 until 17 September 2006, the loop via Heathrow Terminal 4 was closed to allow the connection of a spur line to the now operational Heathrow Terminal 5 station. All underground services reverted to two-way working into Terminals 2 & 3, which again became the temporary terminus; shuttle buses served Terminal 4 from the Hatton Cross bus station. For a brief period in summer 2006, the line terminated at Hatton Cross and shuttle buses also ran to Terminals 2 & 3 while the track configuration and tunnels were altered for the Terminal 5 link from that station. The station at Terminal 5 opened on 27 March 2008 on the same day Terminal 5 opened.

The Piccadilly line bomb resulted in the largest number of fatalities, with 26 people reported killed. Owing to it being a deep-level line, evacuation of station users and access for the emergency services proved difficult. Parts of the line re-opened on 8 July, and full service was restored on 4 August, four weeks after the bomb.

The line from Cockfosters to Heathrow and South Harrow is controlled from a control centre at Earl's Court, which too is shared with the District line controlling the line from Ealing Broadway, Kensington (Olympia) and Parsons Green to High Street Kensington and Tower Hill.[6] Rayners Lane signal cabin is responsible for signalling the Piccadilly line from South Harrow to Uxbridge as well as the Metropolitan line joining at Rayners Lane.[7] The signalling infrastructure is scheduled to be upgraded but at present still runs on an 'obsolete,' system which originated in the 1950s. [8]

Off-Peak services at 21 tph in the core section between Acton Town and Arnos Grove:

6 tph Cockfosters - Heathrow Terminals 4 & 2,3

6 tph Cockfosters - Heathrow Terminals 2,3 & 5

6 tph Cockfosters - Rayners Lane (3 tph continue to Uxbridge)

3 tph Arnos Grove - Northfields

Night Tube:

6 tph Cockfosters - Heathrow Terminals 2,3 & 5

Often late evening services terminate at Oakwood instead of Cockfosters.

Trains also make an additional stop at Turnham Green in the early morning and late evening but do not call there during the main part of the day due to capacity constraints with signalling.

Other services operate at times, especially at the start and towards the end of the traffic day.

Since 16 December 2016, there is a 24-hour Night Tube service on Friday and Saturday nights from Heathrow Terminal 5 to Cockfosters, but not from Uxbridge to Acton Town or the Heathrow Terminal 4 loop.[10]

Like most Underground lines, the Piccadilly line is operated by a single type of rolling stock, in this case the 1973 tube stock, in the standard London Underground livery of blue, white and red. Seventy-nine trains out of a fleet of 86 are needed to run the line's peak service. One unit (166-566-366) was severely damaged by the terrorist attack of 7 July 2005.

The stock was refurbished by Bombardier Transportation between 1995 and 2000.[11] Changes included the removal of transverse seating, strap hangers replaced with grab bars, new floor material and a full repaint into London Underground's corporate livery.[12]

Just beyond Heathrow Terminals 2 & 3 tube station, the line goes into a new section to serve Heathrow Terminal 5 tube station, which opened in March 2008. Half of all Heathrow trains use the loop and serve Terminal 4 and the other half omit Terminal 4 and serve Terminal 5.[13]

Originally the District line; first served by the Piccadilly line 23 October 1933; renamed Hillingdon (Swakeleys) April 1934; the suffix was later dropped; closed when relocated 5 December 1992; re-opened 6 December 1992map 52

Aldwych opened on 30 November 1907 as the Strand tube station. It was at the end of a branch line from the main line at Holborn. An evening through-northbound 'Theatre' train ran until 1910. From 1917 onwards, it was served by a shuttle from Holborn. In the same year it was renamed Aldwych when Charing Cross on the Northern line was renamed Strand. It was temporarily closed in 1940 during World War II to be used as an air-raid shelter. It re-opened in 1946. The possibility of extending the branch to Waterloo was discussed, but the scheme never proceeded.[14] Aldwych was closed on 30 September 1994; the level of use was said to be too low to justify the £1 million in estimated costs of a complete replacement of the lifts. The station is regularly used by film makers.

Park Royal & Twyford Abbey opened 23 June 1903; closed 5 July 1931. Although on the route of the current Piccadilly line, a short distance north of the present Park Royal station, it was never served by Piccadilly line trains. It was opened by the District line, the original operator of the line between Ealing Common and South Harrow, and was closed and replaced by the present Park Royal station before the Piccadilly line started running trains to South Harrow in 1932.

York Road opened 15 December 1906; closed 19 September 1932, between King's Cross St Pancras and Caledonian Road. It has been suggested[15] that this station may be reopened to serve new developments on the nearby Kings Cross railway lands, but this idea is not being progressed at present. The road the station served, 'York Road', has since been renamed 'York Way'.

The Piccadilly line is to be upgraded under the New Tube for London scheme, involving new trains as well as new signalling, increasing the line's capacity by some 24% and reducing journey times by one fifth.[16] Bids for new rolling stock were originally submitted in 2008. However, after the acquisition of Tube Lines by Transport for London in June 2010, this order was cancelled and the upgrade postponed.[17]

LUL then invited Alstom, Bombardier and Siemens Mobility to develop a new concept of lightweight, low-energy, semi-articulated train for the deep-level lines, provisionally called "Evo" (for 'evolution'). Siemens publicised an outline design featuring air-conditioning and battery power to enable the train to run on to the next station if third and fourth rail power were lost. It would have a lower floor and 11% higher passenger capacity than the present tube stock.[18]
There would be a weight saving of 30 tonnes, and the trains would be 17% more energy-efficient with air-conditioning included, or 30% more energy-efficient without it.[19] Siemens Mobility was awarded a £1.5 billion contract in June 2018 to produce the new trains at a planned factory in Goole, East Yorkshire.[20]

The intention is for the new trains to eventually operate on the Bakerloo, Central, Piccadilly and Waterloo & City lines.[21] On current plans, resignalling work on the Piccadilly line will begin in 2019,[22] and new trains are due to enter service in 2023.[20]

^"York Road Station Re-opening – Business Case Analysis"(PDF). Halcrow Group Limited. 2005. p. 6. Archived from the original(PDF) on 26 July 2011. The objective would be to ensure that public transport users travelling from the KCC development would benefit from travelling via York Road Station rather than using King’s Cross St Pancras Station. This in turn leads to the subobjective of providing congestion relief for King’s Cross St Pancras Station.